1. Field of the Invention
The present invention relates to a power steering device utilizing a torsion bar.
2. Description of the Prior Art
When a power steering device utilizing a torsion bar is used, one factor that is felt as a response by the driver at the steering wheel is the torsional resistance of the torsion bar. However, when the steering is in a neutral position, there is almost no torsional resistance of the torsion bar, which results in weaker rigidity of neutrality and hence reduces the stability of the vehicle during a straight travel.
Under such circumstances, various devices have been proposed to provide a preset force to achieve rigidity of neutrality. By way of example, one of such devices will be described below.
FIGS. 9 through 12 show such a power steering device.
In this device, a power cylinder 1 incorporates a piston (not shown) through which an output shaft 2 is penetrated. The piston and the output shaft 2 are linked through a ball nut which is not shown.
Further, a sector gear which is not shown is engaged with the piston to be rotated as the piston moves. The rotation of the sector gear causes a steering wheel linked therewith to turn. This is a so-called integral type power steering device.
A valve case 3 is secured to the power cylinder 1. The base end of the output shaft 2 is rotatably supported by the valve case 3.
The output shaft 2 is hollow, and an end of an input shaft 4 is rotatably inserted therein in a position toward the base end thereof.
The input shaft 4 and output shaft 2 are linked through a torsion bar 5. Specifically, one end of the torsion bar 5 is inserted in the input shaft 4 and secured thereto by a pin 6 that is pierced through the portion where it is inserted. The other end of the torsion bar 5 is secured to the output shaft 2 by a pin which is not shown.
The input shaft 4 and output shaft 2 rotate relative to each other while twisting the torsion bar 5.
A rotary spool 7 is formed to be integral with an outer circumferential surface of the input shaft 4 inserted in the output shaft 2. An inner circumferential surface of the output shaft 2 that faces the rotary spool 7 serves as a rotary sleeve 8. The rotary spool 7 and rotary sleeve 8 are fitted to each other such that they can rotate relative to each other to form a steering valve v.
When the input shaft 4 and output shaft 2 rotate relative to each other, the steering valve v is switched in accordance with the rotating direction to supply operating fluid into one of pressure chambers defined in the power cylinder 1 and to discharge operating fluid in the other pressure chamber to a tank. As a result, the piston moves to rotate the sector gear, thereby applying an assistant force to the wheels linked thereto.
In a power steering device having such a configuration, a spring containing chamber 9 is formed on one end of the output shaft 2 and is blocked from the steering valve v by a seal member 10.
As shown in FIG. 10, the spring containing chamber 9 is substantially in the form of a square surrounded by walls 12, 12 and walls 19, 19 formed by boring the end of the output shaft 2.
The input shaft 4 is inserted in the spring containing chamber 9 in which a pair of spring members 13 are disposed such that they sandwich the input shaft 4, and balls or rollers 21 are interposed between the spring members 13 and the input shaft 4.
A pair of first support grooves 17 are formed in opposite positions on the outer circumferential surface of the input shaft 4. The first support grooves 17 are arranged such that they face the walls 12 of the spring containing chamber 9 to maintain the state shown in FIG. 10 in the neutral state wherein the input shaft 4 and output shaft 2 are not in relative rotation.
As shown in FIG. 12, the spring member 13 is constituted by a flat spring having a flat portion 14, convex portions 15 formed on both sides of the flat portion 14 and slopes 16 continuous with the convex portions 15.
A second support groove 20 which is a V- or U-shaped groove is formed in the middle of the flat portion 14. Further, the ends of both of the slopes 16 are bent to form anchoring portions 18.
The convex portions 15 serve as stoppers to prevent the ball or roller 21 from dropping from the gap between the spring member 13 and input shaft 4 when it comes out the first and second support grooves 17 and 20.
When the spring member 13 is in a free state, the distance between the anchoring portions 18, 18 on both sides thereof is longer than the distance between the walls 19, 19 of the spring containing chamber 9.
Thus, as shown in FIG. 9, when the spring members 13 are placed in the spring containing chamber 9, it is anchored thereto by the anchoring portions 18 being urged into contact with the walls 12 and the walls 19 which are normal to the walls 12 at the corners of the spring containing chamber 9.
The first support grooves 17 are put in a face-to-face relationship with the second support grooves 20 on the spring members 13 in the neutral state wherein the input shaft 4 and output shaft 2 are not in relative rotation. When the spring members 13 are anchored as described above, the distance between the first support grooves 17 and the second support grooves 20 is smaller than that in the state wherein the balls or rollers 21 are interposed therebetween.
When the balls or rollers 21 are interposed between the first and second support grooves 17 and 20, a spring force acts in the direction toward the center of the input shaft 4 to appear as an initial load. The first and second support grooves 17 and 20 are both formed to a small depth that only supports the balls or rollers 21 therein rather than holding them securely.
The operation of this power steering device will now be described.
When the steering wheel is kept in the neutral position, the input shaft 4 and output shaft 2 are in the neutral position shown in FIG. 10. The initial load from the spring members 13 is exerted to the input shaft 4 in the direction of sandwiching the input shaft 4 to act as a preset force through the balls or rollers 21.
Thus, rigidity of the neutral position can be improved to provide stability during a straight travel.
Let us assume that the steering wheel is turned in this state to rotate the input shaft 4 in the direction indicated by the arrow k relative to the output shaft 2.
Then, as shown in FIG. 11, the balls or rollers 21 move over the edges of the supporting grooves 17 and 20 while deflecting the spring members 13 to ride on circumferential portions of the input shaft 4.
Further relative rotation of the input shaft 4 and output shaft 2 causes the balls or rollers 21 to move between the flat portions 14 of the spring members 13 and the circumferential portions of the input shaft 4 in the direction indicated by m in a rolling motion with the spring members 13 kept deflected.
Thus, the steering reaction force generated at this time is a combination of the torsional resistance of the torsion bar 5 and the spring force of the spring members 13.
When the input shaft 4 and output shaft 2 have rotated relative to each other by predetermined amounts, the balls or rollers 21 contact the convex portions 15 of the spring members 13. Therefore, the balls or rollers 21 will not come out from the gap between the flat portions 14 of the spring members 13 and the input shaft 4.
As described above, the relative rotation of the input shaft 4 and output shaft 2 switches the steering valve v to control the operating fluid in to the power cylinder 1, thereby providing an assistant force. When the wheels is steered to a target value as a result of the application of the assistant force, the input shaft 4 and output shaft 2 return the neutral position. At this time, the balls or rollers 21 also roll to return to the positions of the support grooves 17 and 20 and thus they return to the neutral state shown in FIG. 10.
Such a mechanism for applying a preset force may be provided not only on a hydraulic power steering device described above but also on an electric power steering device as shown in FIG. 13.
This device provides an assistant force from an electric motor which is not shown in accordance with the relative rotation of an input shaft 4 and an output shaft 2 incorporated in a housing 41. The magnitude of the relative rotation is detected as the direction and magnitude of the input torque based on which a signal is sent to the electric motor.
It has a mechanism in which the relative rotation of the input shaft 4 and output shaft 2 moves a slider 38 provided on the outer circumference of the input shaft 4 and output shaft 2 in the axial direction thereof, and a torque sensor 37 detects the input torque from the amount of the movement. The slider 38 is coupled to the output shaft 2 by a screw portion 39 and is connected to the input shaft 4 by a spline 40. Therefore, the movement of the slider 38 is suppressed in the rotating direction of the input shaft 4 and is allowed only in the axial direction of the same.
Thus, when the input shaft 4 rotates relative to the output shaft 2, the slider 38 moves in the axial direction thereof.
This device is the same as the device shown in FIG. 9 in that it has a configuration in which a preset force is provided by spring members 13 provided in a spring containing chamber 9 formed at an end of the output shaft 2 such that they sandwich the input shaft 4.
The above-described power steering device can improve rigidity of neutrality by applying a preset force to provide stability of a vehicle during a straight travel.
The balance of the preset force can be maintained because the preset force is applied by the spring members provided on both sides of the input shaft 4. Thus, the rigidity of neutrality does not vary depending on the direction in which the steering wheel is turned.
In such a device, since the spring containing chamber 9 is formed by boring the end of the output shaft 2, the output shaft 2 and the spring containing chamber 9 move integrally.
Thus, when the centering of the output shaft 2 and input shaft 4 is carried out to match their relative positions with the steering valve v in a neutral state of the hydraulic pressure, the relative positions of the spring members 13 as a mechanism for applying the preset force and the input shaft 4 are also determined.
Therefore, when the shaft centering to determine the relative positions of the input and output shafts 4 and 2 is carried out in accordance with the neutral position of the steering valve v, the interior of the spring containing chamber 9 must essentially be in the state as shown in FIG. 10. That is, when the centering of the hydraulic pressure has been carried out, the centering of the mechanism for applying the preset force must have been also completed automatically.
However, it is difficult to achieve both of shaft centering and preset force centering properly through a centering operation on only either of them.
In practice, since priority is given to the centering of the shafts, i.e., the centering of the hydraulic pressure, the preset force may become off balance.
For example, if the spring members 13 and the input shaft 4 are initially in the state shown in FIG. 11 as a result of a slip between their relative positions even though the steering valve v is in the neutral state, the rigidity of neutrality can be off balance to be biased to the left or right.
In order to achieve both of shaft centering and preset force centering using the above-described method, strict control must be performed, for example, on the dimensions of each part, the internal dimensions of the spring containing chamber 9, the flatness of walls, and processing and assembling accuracy of the support grooves 20 on the spring members 13 and the support grooves 17 on the input shaft 4. This obviously leads to an increase in the manufacturing cost.
Further, it is again necessary in the electric power steering device shown in FIG. 13 to secure the torsion bar 5 such that it is not twisted when the output shaft 2 and input shaft 4 are in relative positions adjusted so as to match the axial position of the slider 38 with a zero point of the torque sensor 35. In this case, again, shaft centering to achieve such relative positions of the output shaft 2 and input shaft 4 also determines the relative positions of the spring members 13 as a preset force application mechanism and the input shaft 4.
Therefore, in order to achieve both of shaft centering and preset force centering, strict control must be again performed, for example, on the dimensions of each part, the internal dimensions of the spring containing chamber 9, the flatness of walls, and processing and assembling accuracy of the support grooves 20 on the spring members 13 and the support grooves 17 on the input shaft 4, which results in the same problem as in the example of a hydraulic pressure type power steering device as described above.
It is an object of the present invention to easily achieve both of the centering of input and output shafts to determine their relative positions in a neutral state and the centering of a mechanism for applying a preset force.